The account article treats with the characterizations of the structural and electronic transformations of Pt/C, Pt3Co/C, and Pt3Ni/C cathode electrocatalysts in polymer electrolyte fuel cells (PEFCs) involving Pt charging/discharging, Pt-O bond formation/breaking, and Pt-Pt bond breaking/reformation in cathode potential-jump processes and the rate constants of those transformations in the surface reaction sequences on the cathode electrocatalysts by in situ time-resolved X-ray absorption fine structure (XAFS) method. Spatially heterogeneous issue and mechanism for the Pt oxidation and degradation of Pt/C cathode electrocatalyst layers in PEFCs under the operating conditions, which should be uncovered for development of next-generation PEFCs, were also characterized by a scanning nano-XAFS mapping method and a laminography-XAFS method, respectively for 2D and 3D visualizations of Pt chemical species in Pt/C cathode electrocatalyst layers. These XAFS techniques provided new insights into critical issues affecting the performance and property of practical PEFCs under the operating conditions.